The invention relates to a luminescent material, a luminescent screen and a low pressure mercury lamp comprising such a luminescent screen. The invention also relates to a method for preparing the luminescent material.
Modern low pressure mercury discharge lamps are often equipped with a luminescent screen of the so called three band type. The luminescent screen of such a low pressure mercury discharge lamp comprises three luminescent materials, each of which has a relatively narrow emission band. These emission bands are in the blue, green and red region of the visible spectrum respectively. Such low pressure mercury discharge lamps are described for instance in Netherlands patent 164,697, which corresponds to U.S. Pat. No. 3,397,998. These lamps have a relative high efficacy and a good colour rendering. The luminescent materials that are used in these lamps generally consist of a host lattice activated with ions of an element chosen from the lanthanides. The emission bands of trivalent lanthanide ions is relatively narrow and more in particular the emission bands of Tb.sup.3+ and Eu.sup.3+ are in such a region of the visible spectrum that the lumen/Watt ratio is very favourable. For instance, since the market introduction of low pressure mercury discharge lamps of the three band type, the luminescent material with the emission band in the red region of the visible spectrum that has been used by virtually all manufacturers is yttrium oxide activated with trivalent europium. This luminescent material has a high absorption and a high quantum yield. Additionally the degradation is relatively low. An important disadvantage of this luminescent material, however, is that yttrium oxide is very expensive, the more so since it is often present in the luminescent screen in a relatively large quantity (e.g. 60% by weight).